Our apps don't do anything harmful. On the contrary, we provide you with a
page that describes features
used in our live wallpapers. We have a number of apps published in the official
Play Store.
You can think of the others not placed there as being special because
we wanted to reward visitors of our site with extra apps.

You're going to take a look at a Settings > Security screen.

First, open Settings:

Then proceed to Security:

If you opted-in for Unknown sources before then you're done.

Otherwise please check it:

If you didn't do this prior to downloading our app you would be presented with
a similar screen:

and your device wouldn't trigger an immediate installation screen for the app.
That means you would have to look for the app file manually on your device's storage.
It's not a pleasant job as some Android devices don't have a proper file manager :)